How the Code was Cracked

What Code?

The DNA molecule,
the carrier of the genetic information.

In
1953 James Watson and Francis Crick revealed the structure and
properties of DNA, the molecule that carries our genetic information.
What they discovered was that the blueprint for a human being was
encapsulated in a long string of nucleic acid, arranged in a double
helix, like a twisted rope ladder with three billion rungs. For
this discovery Watson and Crick, together with Maurice Wilkins,
were awarded the Nobel
Prize in Physiology or Medicine in 1962.

But one big question remained unanswered: how
is the information in the DNA strand translated to protein? Among
many others, three scientists, Marshall Warren Nirenberg, Har
Gobind Khorana and Robert William Holley, set their minds on understanding
how the four-letter code of DNA could be translated into the 20-letter
alphabet of amino acids, the building blocks that make up proteins.

Making
Protein from DNA

Genetic
information is located in the nucleus of a cell. It is carried
from one generation to the next through the linear sequence of
nucleotides that make up each of the strands of the DNA helix.
These consist of the four nucleotides adenine, thymine, cytosine
and guanine, abbreviated A, T, C and G. The two DNA strands are
paired in a restricted way: G and C can bind only to each other,
and the same goes for A and T.

When the information is needed to make a protein,
it is first translated to another kind of nucleic acid, called
RNA. RNA is composed much like DNA, but it is single-stranded.
Also, when the strand of DNA letters is translated to RNA, the
T is exchanged for an U (uracil). This RNA is called messenger
RNA (or mRNA).

RNA, a molecule which
resembles DNA, is however single-stranded.

The strand of letters making up the messenger
RNA is then translated to protein in a complex set of reactions
that occur in a structure called a ribosome. First, the amino acids
used for making protein are attached to another kind of RNA, so-called
transfer RNA, or tRNA. This family of clover-leaf shaped molecules
has the capacity to read the genetic code and transform it into
the protein alphabet. Each tRNA binds only to one kind of amino
acid, and each tRNA recognizes a particular set of three nucleotides
in the mRNA strand: triplets that correspond to a particular amino
acid. The protein molecule is then built by adding one amino acid
at a time, using the mRNA as a template.

Attempts to Decipher the
Code

When the structure of DNA was made known,
many scientists were eager to read the message hidden in it. One
was the Russian physicist George Gamow. Many researchers are ”lone
rangers” but Gamow believed that the best way to move forward
was through a joint effort, where scientists from different fields
shared their ideas and results. In 1954, he founded the "RNA Tie
Club." Its aim was "to solve the riddle of the RNA structure and
to understand how it built proteins."

George Gamow, Russian
physicist, founded the "RNA Tie Club" in 1954.

The brotherhood consisted of 20 regular members
(one for each amino-acid), and four honorary members (one for each
nucleotide in nucleic acid). The members all got woolen neckties,
with an embroided green-and-yellow helix (idea and design by Gamow).

Among the members were many prominent scientists,
eight of whom were or became Nobel Laureates. Such examples are
James Watson, who in the club received the code PRO for the amino
acid proline, Francis Crick (TYR for tyrosine) and Sydney Brenner
(VAL for valine). Brenner was awarded the Nobel Prize in Physiology
or Medicine as recently as 2002, for his discoveries concerning
genetic regulation of organ development and programmed cell death.

Early Ideas Sprung from
the "RNA Tie Club"

The members of
the club met twice a year, and in the meantime they wrote each
other letters where they put forward speculative new ideas, which
were not yet ripe enough to be published in scientific journals.

In 1955 Francis Crick proposed his "Adapter Hypothesis,"
which suggested that some (so far unknown) structure carried the
amino acids and put them in the order corresponding to the sequence
in the nucleic acid strand.

Gamow, on the other hand, used mathematics to
establish the number of nucleotides that should be necessary to
make up the code for one amino acid. He postulated that a three-letter
nucleotide code would be enough to define all 20 amino acids.

Not
a Member of the Club

But
it was a non-member of the club who actually deciphered the first
letter of the code, a finding that stunned the scientific community
when the result was presented at a biochemistry conference in Moscow
in 1961.

At the time when the members of the "RNA
Tie Club" were trying to decipher the code, Marshall W. Nirenberg
was also at work in his laboratory at the National Institutes of
Health, outside Washington D.C.

Marshall Nirenberg,
the scientist that deciphered the genetic code in 1961.

Together with his colleague Johann H. Matthaei,
he tried to figure out how the genetic information hidden in the
DNA strand could eventually be read out as protein. They used a
so-called "cell-free" system: in a test tube they put
together all the things they thought were needed for protein synthesis – RNA
template, ribosomes, nucleotides, amino acids, stabilizing agents
and energy.

A Clever Experiment

They carried
out a series of experiments to see what amino acid a particular
nucleotide template gave rise to. Strands of template with a known
combination of nucleotides were run in the "cell-free" system.

They made a very simple nucleic acid, composed
of a chain of only one single, repeated letter – the nucleotide
uracil, or U. Using this nucleic acid, the system produced a protein
that also contained a single letter, but now written in the protein
language: the amino acid F, phenylalanine. By showing that a strand
of U triplets was indeed the template for the amino acid phenylalanine
they cracked the first letter of the code.

This was the result Nirenberg presented in Moscow.
While he was at the conference he got a phone call from Matthaei
(still working at the lab back home) who told him that CCC was
probably the template for the amino acid proline, P.

The experiment which
used uracil (U) as a template produced a protein entirely made
up of the amino acid phenylalanine (F). The first letter of the
genetic code was hence identified.

Solving the Rest of the
Puzzle

Har Gobind Khorana, at the University
of Wisconsin, devised precise and intricate biochemical methods
to produce well-defined nucleic acids, long strands of RNA with
every nucleotide in exact position. The first one he made was a
strand repeating the two nucleotides UCUCUC. This translated into
a strand of amino acids, reading serine-leucine-serine-leucine...
Synthetic RNA were later used to decipher the rest of the genetic
code.

Robert Holley was a chemist at Cornell University,
but learned about protein synthesis during a sabbatical year at
Caltech in California. He discovered the special type of nucleic
acid called transfer RNA, or tRNA for short.

In 1965 Holley was able to work out its exact
structure. This was the first time anyone had established the complete
chemical structure of a nucleic acid that was biologically active.
tRNA turned out to be the missing molecule that Crick had proposed
in his ”Adapter Hypothesis” ten years earlier.

In 1968, seven years after the first letter
of the code was presented in Moscow, Nirenberg, Khorana and Holley
were awarded the Nobel Prize in Physiology or Medicine.

Using the Code Today

The importance
of the research that was awarded the Nobel Prize in Physiology
or Medicine in 1968 cannot be overestimated. Cracking the code
of life paved the way for a tremendous boom in molecular biology,
enabling scientists to put together strings of DNA and RNA to produce
selected proteins.

One example where this technique is used is in
the production of pharmaceuticals. The DNA that encodes the protein
you want is synthesized and put in bacteria. A new copy of the
desired protein is produced every time the bacterium divides. Since
an E. coli bacteria can produce approximately 17 million
daughter cells during an 8-hour working day (and bacteria work
24 hours a day, 7 days a week), the production is very efficient.
In this fashion it is now possible to make many useful proteins,
for example insuline (to treat diabetic patients) and different
coagulation factors that are needed by patients suffering from
hemophilia.